I'm not sure about the motherboard, but I've found these articles to be very helpful:
Best gaming CPU for the money december '12:
http://www.tomshardware.com/reviews/...lock,3106.html
Best gaming GPU for the money december '12:
http://www.tomshardware.com/reviews/...view,3107.html
In accordance with the links and what I've heard I'd recommend the i5-3450 (195$) for a non overclock build and the HD7850 (185$) as the GPU. If you're ready to spend a little more and want to overclock, the i5-3570K (215$) should be one of the best processors to get at the moment at a fair price, and you could also upgrade to a HD7870 (240$) or a GTX660 (which is not shown in the article but still a nice GPU).
I'm not really familiar with the performance of the HD6850 but the above mentioned GPU's are nice deals.